Spatial Diversity In Wireless Communications

Spatial diversity is one of those fundamental technologies used in wireless communications
(cellular networks, Wi-Fi, satellite communications, and broadcasting) that does not get much exposure. The technology is used to combat fading and improve signal quality, enabling reliable communication links, especially in challenging environments characterized by obstacles, interference, or long propagation distances. Let’s take an introductory look. 

Spatial diversity exploits the spatial dimension of wireless channels by deploying multiple antennas at either the transmitter or receiver, or both. By leveraging spatial separation between antennas, spatial diversity techniques minimize the effects of fading, which result from signal attenuation, reflections, and scattering in multipath propagation environments. Through the simultaneous reception of multiple independent copies of a transmitted signal, spatial diversity enhances the likelihood of receiving at least one strong signal, thus improving the overall reliability of communication links.

 

There are three key methods involved - Selection Diversity, Maximal Ratio  Combining (MRC) and Equal Gain Combining (EGC).

 

Selection Diversity: In selection diversity, multiple antennas are strategically placed to receive the same signal, and the antenna with the highest received signal strength is chosen for further processing. This technique is relatively simple to implement and offers improved diversity gain, particularly in scenarios with moderate to severe fading.

 

Maximal Ratio Combining (MRC): MRC combines signals from multiple antennas with different complex weights, determined based on the channel conditions. By weighting each received signal based on its signal-to-noise ratio (SNR) and combining them coherently, MRC maximizes the received signal power, thereby enhancing the overall signal quality and reliability.

 

Equal Gain Combining (EGC): EGC employs a simpler approach by combining signals from multiple antennas with equal weights. While less complex than MRC, EGC provides diversity gain by mitigating the impact of fading through signal averaging.

 

Spatial diversity offers an effective mechanism to combat fading and enhance signal reliability. Through the strategic deployment of multiple antennas and the application of diverse combining techniques, the technology improves data transmission across a wide range of environments and applications.

Wireless Shift: LTE Macrocells -> Small Base Stations

I’ve written about femtocells here in the past - basically micro-cell antennas that users attach to their broadband network. They are used to fill in areas where coverage is weak or does not exist. It looks like this idea is catching on. With increasing traffic demands along with OPEX and CAPEX savings in mind we’re starting to see a shift in the deployment of small range cell base stations (what some are calling simply “small cell”). 

Looking at a recent ABI Research report that examines the ecosystem and outlook for LTE base stations titled “The LTE Base Station Market,” we see the uptake of small cells and remote radio heads (RRHs) as elements of the distributed base station. 

Here’s some detail from the report:

• Operators will initially deploy small cell equipment as in fills on the pico and microcell layers, but will quickly transition to deploying them as a fundamental part of a network rollout. 
• The number of LTE small cells sold (127,000) will surpass the number of LTE macrocells, forecast at 113,000, as early as 2014. 
• Semiconductor suppliers are positioning themselves to participate in this market with TI, Freescale, Cavium, Mindspeed, and DesignArt among the manufacturers offering new “base station-on-a-chip” SoCs.
• However, LTE base station revenues will continue to be dominated by macro base station revenue with small cell revenue of $1.09 billion representing only 5.2% of the total revenue of $20.86 billion in 2014 and growing to $4.44 billion or 23.9% of the total $18.60 billion LTE base station market by 2016.
• Equipment manufacturers have been quick to respond to this shift in RAN (Radio Access Network) architecture. Ericsson acquired BelAir networks as part of its “HetNet” initiative, Nokia Siemens Networks announced Flexi Zone, Alcatel-Lucent continues to expand its lightRadio™ portfolio and Huawei has announced its AtomCell products.

Nick Marshall, principal ABI Networks Analyst, comments regarding the report, “This mobile broadband-driven data storm is stretching traditional macrocell network capacity to the limit and driving the move to heterogeneous networks.” Nick continues, saying, “These base station baseband SoCs (System on a Chip) are among the most complex ICs on the market today and raise the bar in terms of complexity.”

Are Wired Keyboards Secure?

Last week Martin Vuagnoux and Sylvain Pasini, doctorate students with the Security and Cryptography Laboratory at the Ecole Polytechnique Fédérale de Lausanne in Switzerland, posted an interesting piece titled Compromising Electromagnetic Emanations of Wired Keyboards.

Vuagnous and Pasini tested 11 different wired mechanical keyboards (PS/2, USB and laptop) purchased between 2001 and 2008 and used four different attack methods on each. They claim they were able to fully or partially recover keystrokes electromagnetically at distances up to 20 meters including through walls. They've posted two videos at Dailymotion.com demonstrating how they were able to collect keystrokes. The first shows a Logitech keyboard with a PS/2 connector attached to a laptop. A one meter wire cable was used as an antenna and placed one meter away from the keyboard. The monitoring system was able to pickup the phrase "trust no one" when it was typed on the keyboard:

The second video shows how a larger antenna can be used to snag keystrokes though a wall:

Vuagnous and Pasini conclude that wired computer keyboards sold in the stores generate compromising emanations (mainly because of the cost pressures in the design). Hence they are not safe to transmit sensitive information. No doubt that our attacks can be significantly improved, since we used relatively inexpensive equipments. They go on to say more information on these attacks will be published soon, with a paper currently in a peer review process for a conference.

It will be interesting to see if others can duplicate this work - these would make nice classroom experiments.